A regular mortise lock, unit lock or latch generally utilizes a latch bolt that has a rectilinear motion to lock up with or unlock from a striking plate mounted on a door frame so as to lock or unlock a door.
The lock which locks a door by means of the rectilinear motion of a latch bolt may be easily destroyed by burglars through violence. In a regular one-step locking device, the single latch bolt is driven only a limited distance into the notch of a striking plate mounted on a door frame. Through severe impact force against the door, the door panel and the latch bolt will be easily deformed, and the latch bolt will be pushed out of the associated striking plate to unlock the door (This is commonly seen in TV programs). In a regular two or three-step locking device (which comprises two or three latch bolts), the door still cannot be well protected against violence. Although the latch bolts are inserted more deeply in the notches of a striking plate, the range of the latch bolts into the notches of a striking plate is still limited by the thickness of a door frame. If a regular two or three-step locking device is used to lock a door, a burglar may hit the door to open it after having inserted a crowbar into the gap between the door panel and the associated door frame to pry the door panel in or out.